Biochar from co-pyrolysis of spent coffee ground with Leptolyngbya sp. KC 45 biomass and residue for energy and agricultural utilization

• Higher LT or LTR in SCG boosted biochar yield, surface area, and pore volume. • Co-pyrolyzed biochar had smaller mesopores compared to pure SCG biochar. • pH (6.2–7.6), EC (1.01–1.90 dS/m), and moisture content were within soil standards. • C/N ratios (14.53–22.64) of specific blends suggest suitability for soil amendment. • SCG biochar (33.61 MJ/kg) has a heating value similar to coal, suitable for fuel. As the global population continues to grow, the demand for resources in agriculture, energy, and industry is also increasing, along with higher waste production. This study explored the potential of using spent coffee grounds (SCG) from local coffee shops in Chiang Mai, Thailand, for co-pyrolysis with either Leptolyngbya sp. KC 45 (LT) or its residue (LTR)—materials containing valuable antioxidants and pigments used in food, cosmetics, and medicine. The study investigates the chemical properties, quality characteristic, and morphology of the biochar. Various co-pyrolysis ratios of SCG to either LT or LTR (1:0, 2:1, 1:1, 1:2, 0:1) were tested, with 50 g of the mixture pyrolyzed at 500 °C in a fixed-bed reactor, using a heating rate of 5–10 °C/min, nitrogen flow of 100 ml/min, and a 1-hour holding time. Results showed that higher LT or LTR proportions increased biochar yield. Pure SCG biochar had a high heating value of 33.61 MJ/kg, confirming its fuel quality. Co-pyrolyzed biochars exhibited mesoporous structures and improved properties such as pH (6.2–6.5), electrical conductivity (1.01–1.45 dS/m), and C/N ratio (14.53–22.64), indicating their potential as soil amendments. This research contributes to reducing environmental waste by adding value to waste materials.